Re: Mixed bag with the Air Core Antenna add-on to Tecsun PL310

jim_kr1s <jkearman@...>

--- In ultralightdx@..., "Chris Knight" <chris@...> wrote:

The MW band is a pretty large swath of real estate for a single fixed coil to cover.
Everything was swell until they added X band! But I think SiLabs guarantees it will tune the US AM BC band with one coil. If the range of inductances is correct, that varactor has a wider range than any commercial discrete varactor I know of.

By virtue of the higher reactance at 1700 kHz, you're going to get more Q up there, so if the varactor just wheezes its way to the top, it should work pretty well. A given capacitance range will always tune the same number of octaves no matter the inductance. Assuming the chip can tune 530-1710, a 3.23 ratio, it should almost cover the 153-513 kHz LW band. You'd have to sacrifice a few kHz, but chances are the varactor can do a little better. But -- the inductance to resonate with 350 pF at 153 kHz is a whopping 3092 uH! No wonder the little guys don't hear well on LW. Even with a larger winding you'd have to add some loading inductance to make it resonate. You'd probably need about 1500-2000 uH, and a high-Q coil would be rather large. I'm thinking of how large those old 2.5-mH (2500-uH) National rf chokes were, and they had lousy Q by design. (By the way, those old chokes are a good source of fine Litz wire, if you unwind them carefully.)

Here's where your plug-in coils would solve the problem! I think you could get 3000 uH on a 7.5-inch rod.

I also think Tecsun should fly us all over for some Szechuan chicken, a walk on the Great Wall, and a consult on their next model!


Jim, KR1S

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